The present invention concerns a friction bearing for the rotor pivot of the liquid counter in a cryogenic medium.
The known liquid counters for cryogenic media are subject to high temperature loads. The rotor in the liquid counter, by which the liquid flow is measured quantitatively, is supported by its rotor pivot in a friction bearing usually made of plastic material. This friction bearing proper is inserted in a metallic holder by friction contact, namely by press fit or by gluing in.
The fastening of the friction bearing, which is located inside the liquid counter by press fit, is totally unsuitable when the liquid counter is used in cryogenic media because the friction bearing (made of plastic) is subject to a considerably greater shrinkage in the outside diameter than is the metallic holder enclosing it. This is due to the fact that the plastic materials used for the friction bearings have a thermal expansion coefficient in the order of magnitude of 10.sup.-4 to 10.sup.-5 .degree. C..sup.-1. The expansion coefficient of the material of the metallic holder is substantially smaller, so that the change in length of the plastic friction bearing due to the cooling of these components by a cryogenic medium is about 3 to 20 times as large as that of the metallic holder.
If the differences in the thermal expansion coefficients (of the friction bearing and of the metallic holder) are taken into consideration in the design, i.e., if the bearing is dimensioned for the operating state (i.e., for a temperature of approximately -200.degree. C.), the fit ratio at normal temperature (20.degree. C.) imposes a high external tension load on the plastic material. Since plastics are unable to take up major compressive forces, plastic flow begins under load, which leads to the loss of the intended bearing geometry as well as of the desired fit ratio. The temperature and load cycles occurring during the intermittent use of the liquid counter moreover lead to material fatigue. Depending on the type of plastic used for the friction bearing, cracks are generated, which grow into crumbled spots due to the penetration of liquid, particularly in the especially suitable multicomponent materials (e.g., metal-impregnated synthetic carbon, PTTE compounds, etc.).
The connection of the friction bearing with the metallic holder holding it by gluing, which is otherwise a usual method, is impossible in connection with cryogenic media, especially when a liquid oxygen flow is measured by the liquid counter. This is due to the fact that there are no adhesives available which are resistant to oxygen and are also elastic down to temperatures as low as -200.degree. C. and thus able to neutralize the fit play occurring during frequent load cycles.
The radial and axial fastening of the friction bearing in the metallic holder (housing) is, however, unavoidably necessary, because the mountings used in high-precision liquid counters are required not only to have low-friction operation and a constant friction coefficient, but also constant frictional forces during the entire lifetime of the bearing.